The development of assays for the murine and human megakaryocytic progenitor cell (CFU-M) has made it possible to examine early events in this cell lineage. We propose to study the regulation of murine megakaryocytopoiesis both in vivo and in vitro, and extend these studies to abnormalities of plateles and megakaryocytes in human disease. In vivo regulatory studies will focus on changes in CFU-M number and cycle characteristics in animals hypertransfused with platelets or made thrombocytopenic by antibody or radiation, and of the effect of transfusion on plasma from animals made thrombocytopenic or thrombocytotic. In order to study the effects of these manipulations on endoreduplication of megakaryocytes, methods to measure the ploidy of individual cells in cultured megakaryocytic colonies will be developed. In vitro studies will focus on the purification of the mekaryocytic colony stimulating factor (M-CSF) derived from murine spleen cells. The cell(s) of origin and mechanism of production of M-CSF will be explored by cell separation and immunologic manipulations of the spleen cells. Finally, efforts to produce M-CSF for human colonies will be made in order to develop a more quantitative assay for human CFU-M. Immunofluorescent methods will be used to identify human colonies. A clonal assay for human CFU-M will be used to analyze abnormalities of regulation or of progenitor cells in thrombocytopenic and thrombocytotic states.